The present invention relates to a carrier wave regenerating circuit. It is used in digital transmission and more particularly in the case where, after having been transmitted by the modulation of an electromagnetic wave, information is restored by coherent demodulation. The field of application of the invention is consequently very wide and covers the modems of data transmission, radio links, space communications systems and optics (in the case of heterodyne connections).
The coherent demodulation of a wave presupposes that the receiver knows the frequency and phase of the transmission carrier wave. To this end, a demodulation wave is produced by a voltage-sensitive oscillator or VSO and the control voltage of this oscillator is supplied by a phase comparator, which supplies an error voltage .epsilon.(.phi.), which is a function of the phase deviation .phi. between the modulated wave and that which is produced by the oscillator. Bearing in mind the fact that the transmission carrier wave can be modulated in amplitude and/or phase, the phase comparator structure is dependent on the modulation used.
FIG. 1 is a general diagram of a carrier wave regenerating circuit. A VSO 10 has a control input 11 and an output 12 and supplies a wave applied to a member 20 having another input 21, which receives the modulated wave and an output 22 supplying an error signal .epsilon.(.phi.). This signal is applied on the return to the VSO input 11.
The characteristic .epsilon.(.phi.) of the phase comparator 20 must have the following properties:
(i) it breaks down with the phase deviation .phi. and changes sign with .phi., PA1 (ii) it is cyclic of cyclic 2.pi./M in which M is the order of symmetry of the modulation, PA1 (iii) it only breaks down once per cycle.
To provide a better understanding of the features of the invention, it is worth referring to the main known types of structures used for regenerating a carrier in this way. The case of phase modulation or PM will firstly be referred to and then that of amplitude modulation in its variant with two waves in quadrature or QAM.
In phase modulation systems, a number of loops are known which will be briefly described in connection with FIGS. 2 to 4. It is assumed that this modulation involves M phase states, M being in general equal to 4, 8 or higher.
In a first type of loop, the frequency, i.e. the phase is multiplied by M, which eliminates the modulation and the VSO is made dependent on the thus produced wave. A loop according to this principle is shown in FIG. 2. It comprises two multipliers by M, respectively 24 and 26, both connected to a phase comparator 28.
In a second type of loop, called the COSTAS loop, an error signal of form sin M.phi. is formed by working on the base band, i.e. on the demodulated signal. FIG. 3 shows the corresponding structure. It comprises a demodulator 30 and a base band processing signal 32 able to process the error signal in question.
In a third type of loop, the phase of the signal is compared with that of its remodulated version. Such a loop is shown in FIG. 4. In addition to the VSO 10 and demodulator 30, it comprises a decision circuit 34 for regenerating the information contained in the demodulated signal, said circuit being followed by a remodulator 35. A comparator 36 receives the modulated wave and the remodulated wave.
In another type of loop, which is not illustrated because it is very similar to the previous loop, the remodulation processing takes place in the base band, as for the COSTAS loop.
Finally, in the so-called digital COSTAS loop, the error signal formed is of the type sgn(sin M.phi.) in which sgn represents the sign of the quantity which follows, this notation also being used throughout the remainder of the description. The latter loop has the advantage of a simple construction (the sign multipliers being formed by EXCLUSIVE-OR logic gates), whilst having good performance levels (the corresponding ideal phase comparator slope d.epsilon./d.phi. would be infinity in the absence of noise.
With regards to the carrier regenerating loops, usable with the second type of modulation, called QAM, they are illustrated in FIGS. 5 to 7. QAM 16 can be considered as the superimposing of two waves in quadrature, each modulated in accordance with four amplitude levels. The 16 possible modulation states can be represented on a vector diagram, which is also called a constellation and which is shown in FIG. 5. In the latter, each axis represents a wave, whose amplitude can assume any one of four amplitude levels 3, -1, 1 and -3 and each state is represented by a cross.
Several loops functioning in QAM 16 are known. Certain of these are of the remodulation type, cf FIG. 4, but having a more complex structure, because QAM is more complicated than PM.
To simplify the construction of such a loop, instead of comparing the signal with its remodulated version, it is compared with the signal with four phase states corresponding to the mean value of the states of each quadrant. These four mean states are shown in the diagram of FIG. 5 by a circle in each quadrant. Although it is simpler than the previous loop, the present loop introduces a "modulation noise", because the error signal is only cancelled out on average for .phi.=0.
According to another type of loop, specifically the digital COSTAS loop, the QAM signal 16 is processed as if it was a PM signal 4. Although simpler than the previous loop, this loop unfortunately has the deficiency of modulation noise.
According to another type of loop, called the digital COSTAS loop with selective gate, the error signal is only formed on the basis of the diagonal states of the vector diagram, corresponding to phases of 45.degree., 135.degree., 225.degree. and 315.degree., which are those of a PM 4. Thus, the modulation noise is removed, by eliminating the contribution of the non-diagonal states to the error signal by means of a selective gate. This loop is described in the article by Horikawa et al entitled "Design and performance of a 200 Mbit/16 QAM digital radio system" published in the Journal IEEE Trans COM, Dec. 27th 1979, p. 1953. At present, this is the loop with the best performance features.
With regards to QAM 32 and QAM 64 modulations, no structure has as yet been proposed, apart from the extension to these modulations of the methods used for QAM 16.
All the prior art circuits have a number of disadvantages.
(1) In PM 8, the digital COSTAS loop leads to a very heavy structure, as can be gathered from FIG. 6. The loop comprises four demodulators 41, 42, 43 and 44 working with waves phase-shifted by 45.degree. from one another, two adders 46, 47, two subtracters 48, 49, 8 threshold comparators 51 to 58, 7 logic EXCLUSIVE-OR gates 61 to 67 four flip-flops 71 to 74. In a simpler variant, only two modulators functioning in quadrature are used, with one adder, one subtracter and two supplementary comparators. This type of structure is described in the article by HOGGE published in the Journal IEEE Com 26, No. 5, May 1978, pp. 528-533.
With such a loop, an error signal is formed which, for PM 4, is of form: EQU .epsilon.(.phi.)=sgn(X)sgn(Y)sgn(X-Y)sgn(X+Y)
(2) In QAM 16, the following disadvantages are encountered. In the case of the partial remodulation loop and the conventional digital COSTAS loop, a modulation noise appears and the error signal is not cancelled out with the phase error except on an average basis. In the case of the digital COSTAS loop with the selective gate, the system may not tolerate a situation in which only the non-diagonal signals are transmitted. Even in the case of digital COSTAS loops, it is necessary to carry out subtractions of the analog signals, which causes constructional problems.